The invention concerns improvements made at the time of filling containers of plastic material, when such operation includes at least one stage during which a significant difference in pressure occurs between the interior of the container and the internal environment in the filling installation, and when the operation is done when the containers are hot and have areas that are more or less malleable. This is the case when the filling phase of the container with any product is preceded by placing them under depression (more or less significant vacuum) from the interior of the container, particularly while being filled with beer, or during overpressure when filling with a gaseous liquid, and when the containers are immediately filled after manufacture by blow molding or extrusion, blowing of a blank. It concerns a procedure and installation for its embodiment.
The filling of a container with any product may sometimes be preceded by placing the interior of the container under vacuum or pronounced depression, for example to replace the air found in it by another medium, to avoid spoiling the product which will be finally packaged in the container. For example, this is the case in the filling of oxide-sensitive products such as beer, certain fruit juices and others: any trace of the oxidizing product must be removed, and in this case it must be rendered inert, for example with nitrogen.
The filling of a container, such as a bottle, with gaseous liquid classically consists of a phase of creating overpressure in the interior of the bottle with a gas, typically carbon dioxide, followed by a phase of filling with liquid, and a phase of depressurization to remove excess gas, while maintaining a certain gas pressure inside.
The pressure difference causes problems in plastic containers, when filling is attempted a few seconds after the containers came out of the blowing mold and are still hot, as is the case in the so-called in-line filling installations.
With these containers, it is not possible to put them under depression before filling, without causing deformation by collapse or crushing of the containers.
With the same type of containers, filling with gaseous liquids creates the following problem: the overpressure phase of the containers before filling makes them burst or causes irreversible deformation.
The deformations or bursting affect the body of the containers, but one can see deformations affecting more particularly the bottom of the containers (a phenomenon called "stress cracking" in professional language).
These phenomena are due to the fact that the plastic container is obtained by blowing of a blank (preform, parison, intermediate container), before bringing it to its blow temperature, therefore softened by heat. When the container comes out of the blowing mold, it still has more or less hot zones, which are therefore more or less malleable. In general, these are the zones that are extruded the least during blowing, and which become cold more slowly for various reasons; and the bottom is one of the zones which are the least extruded. However, if, during the time the pressure difference is present, the temperature exceeds the softening temperature even more, a deformation may occur because of the mechanical force exercised on these zones by internal pressure (overpressure or depression)
It also happens, although more rarely, that bursting or deformations occur when filling is done without creating depression or prior overpressure with a gas, and the pressure at which the liquid is introduced or, more generally, the pressure of the filling product is high.
Indeed, plastic containers and therefore their blanks are sized to withstand internal pressure values (overpressure or depression) necessary for the filling or the preservation of the products after closing, when the material is stabilized and therefore cooled.
This is why, until now, all filling attempts under the aforementioned conditions, with plastic containers which still have zones at a temperature higher than the softening temperature, and sized to withhold the same conditions when the material is stabilized, have failed, and in-line filling was not applied at industrial scale.
A possible solution has been to oversize the containers in order to compensate their formation by surplus material. This solution, however, is not realistic for several reasons, among which are: on the one hand, it is in contradiction with the current trend to make the containers longer, for reasons of cost of materials; on the other hand, the containers contained are rather unaesthetic; in addition, paradoxically, the surplus of material makes the containers fragile when stabilized; finally, the surplus material necessary for filling, becomes useless when the containers are cooled.